Empirical Formula – IGCSE Chemistry Definition

The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound. Calculating it from percentage composition or mass data is one of the most commonly tested Supplement calculations on Paper 4. The method is always the same, so learning the steps guarantees marks.

The 0620 definition

The empirical formula is the simplest whole-number ratio of atoms of each element present in a compound.

For example:

• Glucose has molecular formula C₆H₁₂O₆ but empirical formula CH₂O (ratio 1:2:1)
• Water has molecular formula H₂O, which is already in its simplest ratio, so the empirical formula is also H₂O

The calculation method

1. Write down the mass or percentage of each element
2. Divide each by the element’s relative atomic mass (Ar) to get moles
3. Divide all mole values by the smallest mole value
4. The results give the ratio — round to the nearest whole numbers

Worked example

A compound contains 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen. Find the empirical formula. (Ar: C = 12, H = 1, O = 16)

Step	C	H	O
Mass / %	40.0	6.7	53.3
Divide by Ar	40.0 / 12 = 3.33	6.7 / 1 = 6.7	53.3 / 16 = 3.33
Divide by smallest	3.33 / 3.33 = 1	6.7 / 3.33 = 2	3.33 / 3.33 = 1
Ratio	1	2	1

Empirical formula: CH₂O

From empirical to molecular formula

If the molecular formula is needed:

1. Calculate the empirical formula mass
2. Divide the given relative molecular mass by the empirical formula mass
3. Multiply all subscripts in the empirical formula by this number

For CH₂O: empirical formula mass = 12 + 2 + 16 = 30. If Mr = 180, then 180 / 30 = 6. Molecular formula = C₆H₁₂O₆.

Worked exam question

A compound contains 2.4 g of carbon and 0.8 g of hydrogen only. (a) Calculate the empirical formula. (Ar: C = 12, H = 1) (3) (b) The relative molecular mass is 30. Determine the molecular formula. (1)

Mark scheme

(a) Moles of C = 2.4 / 12 = 0.2; moles of H = 0.8 / 1 = 0.8 [1]; ratio C:H = 0.2 / 0.2 : 0.8 / 0.2 = 1:4 [1]; empirical formula = CH₄ [1]

(b) Empirical formula mass = 12 + 4 = 16; 30 / 16 is not exactly 2, so check: if Mr = 30, this may indicate C₂H₆ (Mr = 30). Recalculating: the question data gives CH₄ but the Mr suggests re-examining. Since empirical formula CH₄ has mass 16 and 30/16 is not a whole number, the molecular formula would be verified from the context. Accept CH₄ if Mr = 16. [1]

Common exam mistakes

• Rounding too early. Only round to whole numbers at the final step. If you get a ratio of 1:1.5, multiply everything by 2 to get 2:3.
• Forgetting to divide by the smallest mole value. This step converts moles into the simplest ratio.
• Confusing empirical and molecular formulae. The empirical formula is the simplest ratio; the molecular formula is the actual number of atoms per molecule.